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2.2 The Hubble Redshift-Distance Relation

Expansion that preserves homogeneity requires that the mean rate of change of separation of pairs of galaxies with separation R varies as the Hubble law,

Equation 10 (10)

The redshift-distance relation for type Ia supernovae gives an elegant demonstration of this relation ([1], [2]). Arp ([21], [22]) points out that such precision tests do not directly apply to the quasars, and he finds fascinating evidence in sky maps for associations of quasars with galaxies at distinctly lower redshifts. But there is a counterargument, along lines pioneered by Bergeron [23], as follows.

A quasar spectrum may contain absorption lines characteristic of a cloud of neutral atomic hydrogen at surface density SigmaHI gtapprox 3 x 1017 atoms cm-2. If this absorption system is at redshift z ltapprox 1 a galaxy at the same redshift is close enough that there is a reasonable chance observing it, and with high probability an optical image does show a galaxy close to the quasar and at the redshift of the absorption lines ([24], [25]). Also, when a galaxy image appears in the sky close to a quasar at higher redshift then with high probability the quasar spectrum has absorption lines at the redshift of the galaxy. We have good evidence the galaxy is at the distance indicated by its redshift. We can be sure the quasar is behind the galaxy: the quasar light had to have passed through the galaxy to have produced the absorption lines. If quasars were not at their cosmological distances we ought to have examples of a quasar appearing close to the line of sight to a lower redshift galaxy and without the characteristic absorption lines produced by the gas in and around the galaxy.

Arp's approach to this issue is important, but I am influenced by what seems to be this direct and clear interpretation of the Bergeron effect, that indicates redshift is a good measure of distance for quasars as well as galaxies.